Urinary hydroxy polycyclic aromatic hydrocarbons (OH-PAHs) are commonly used in biomonitoring to assess exposure to polycyclic aromatic hydrocarbons (PAHs). Similar to other biologically non-persistent chemicals, OH-PAHs have relatively short biological half-lives (4.4-35 hours). Little information is available on their variability in urinary concentrations over time in non-occupationally exposed subjects. This study was designed to (i) study the variability of 9 urinary OH-PAH metabolite concentrations over time and (ii) calculate sample size requirements for future epidemiological studies based on spot urine, first morning void and 24-hour void sampling. Individual urine samples (n = 427) were collected during one week from 8 nonoccupationally exposed adults. We recorded the time and volume of each urine excretion, dietary details, and the driving activities of the participants. Within subjects, the coefficients of variation (CV) for the wet-weight concentration of OH-PAHs in all samples ranged from 45% to 297%; creatinine adjustment reduced the CV to 19-288% (p < 0.001; paired t-test). The simulated 24-hour void concentrations were the least variable measure, with CVs ranging 13-182% for the 9 OH-PAHs. Within-day variability contributed on average 84%, and between-day variability accounted for 16% of the total variance of 1-hydroxypyrene (1-PYR). Intraclass correlation coefficients (ICC) of 1-PYR levels were 0.55 for spot urine samples, 0.60 for first-morning voids, and 0.76 for 24-hour voids, indicating a high degree of correlation between urine measurements collected from the same subject over time. Sample size calculations were performed to estimate the number of subjects needed for detecting differences in geometric mean at a statistical power of 80% for spot urine, first-morning, and 24-hour void sampling. These data will aid in the design of future studies of PAHs and possibly other biologically non-persistent chemicals and the interpretation of their analytical results.